My boyfriend wants to involve another man in our sex life

My boyfriend of 18 months has let me know he fancies involving another man in our sex life. He points out potential candidates wherever we go, and is always suggesting various friends to check if I'm interested. It's not that I'm totally against the idea – in fact it arouses me just thinking about it – but I'm afraid of what might happen in the aftermath.

You are right to be concerned. This may be a test of your fidelity, so try to unearth your boyfriend's true motives before agreeing to it. Even if his interest is purely erotic, a ménage à trois is a difficult thing to achieve with impunity. It requires careful negotiation between all three parties, and there should be some rules agreed on by all.

For example, what kind of STD/pregnancy protection will be used? How would you feel if there was also sexual contact between your boyfriend and the other man? Is any area of anyone's body off-limits? Is any particular sexual style or behaviour unacceptable to any one individual? You and your boyfriend are familiar with each other's needs and limitations, but this third person would be a sexual stranger.

All possible scenarios need to be carefully thought out with a clear head. But on the positive side, there is certainly the potential for an extremely exciting experience – with endless erotic possibilities.

• Pamela Stephenson Connolly is a clinical psychologist and psychotherapist who specialises in treating sexual disorders.